Oriented cholesteric layers usually exhibit a molecular arrangement in which the axis of the helix formed by the mesogenic groups is aligned perpendicular to the two outer surfaces of the film. The pitch of the helix is constant over the layer thickness. Such films are optically active and are distinguished by the fact that unpolarized incident light in the region of the reflection wavelength is split into two circular-polarized part-beams, one of which is reflected and the other of which is transmitted. The two light components differ through their direction of rotation, which is determined by the chiral species present in the optically active layer. The type and proportion of the chiral species of such cholesteric liquid crystals determine the pitch of the twisted structure and thus the wavelength of the reflected light. The twist of the structure can be either left-handed or right-handed. The reflection color is connected to the pitch of the twisted structure referred to as p, and the mean molecular refractive index n. The following relationship applies for the central wavelength of the reflected light: EQU .lambda.=n.times.p
The reflected light has a band width given by the difference between the two refractive indices: EQU .DELTA..lambda.=p.times.(n.sub.e -n.sub.o)
n.sub.o and n.sub.e denote the refractive indices of the ordinary and extraordinary beams, while the following relationship applies to the mean refractive index n: EQU n=1/2(n.sub.o +n.sub.e)
Polymers having a cholesteric phase usually have a refractive index difference .DELTA.n of from 0.1 to 0.2, giving a reflected light band width of &lt;100 nm for light in the visible spectral region. The band widths are frequently only 30-50 nm, in particular for light in the blue region of the optical spectrum.
A combination of these cholesteric films with a quarter-wave retardation element enables the generation of linear-polarized light. Such combinations using cholesteric polymers are described, for example, in R. Maurer et al. under the title "Polarizing Color Filters made from Cholesteric LC Silicones", SID 90 Digest, 1990, pp. 110-113.
Cholesteric layers used in the field of filters and reflectors require adjustment of the optical properties over a broad spectral region, where it should be possible to adjust the mean wavelength of the reflection band and the width of the band as flexibly as possible. It has hitherto only been possible to adjust the band width in a narrow range due to the limited potential for changing the refractive index by molecular optimization. A polymer having this limited band width is usually established before crosslinking by a combination of suitable monomers and a suitable chiral species and fixed by crosslinking. After crosslinking, correction is usually not possible.
In many instances, however, there is a need for optically active layers whose band width covers the entire visible spectral region of light. For this, band widths of at least 250 nm are required. This is achievable by a combination of a number of layers of cholesteric polymers with different pitch. One proposal in this respect is given in R. Maurer, as cited above. The production of such multiple layers, however, is complex and expensive.
Theoretical considerations by R. Dreher, Solid State Communications, Vol. 12, pp. 519-522, 1973, S. Mazkedian, S. Melone, F. Rustichelli, J. Physique Colloq. 36, C1-283 (1974), and L. E. Hajdo, A. C. Eringen, J. Opt. Soc. Am. 36, 1017 (1976), show that a helical layer structure whose pitch changes in a linear manner over the layer thickness of the film should have the ability to reflect light in a broad band range. R. S. Pindak, C. C. Huang, J. T. Ho, Phys. Rev. Lett. 32, 43 (1974), describe the generation of a pitch gradient in cholesteryl nonanoates by means of a temperature gradient. It is a disadvantage of the process described that, due to the generally good thermal conductivity of polymers in thin films, high temperature gradients can only be produced with difficulty. The pitch gradient produced is, thus, only visible at the particular temperature.
EP 0 606 940 A2 (Broer et al) describes the generation of a helix with continuously varied pitch by diffusion of different monomers (chiral and achiral) during simultaneous and slow crosslinking. The use of an additional dye to generate an axial gradient in the light intensity is necessary for relatively large band widths. In addition to the long exposure time to UV light, the additional dye restricts the spectral band width in the region of short wavelengths. A further disadvantage of this process is that, due to the restriction to certain monomers and chiral species, little flexibility is possible regarding optimum liquid-crystalline properties and preparative handling.